In electroerosion wire-cutting, a thin continuous wire or filamentary electrode, typically of a thickness of 0.05 to 0.5 mm, is continuously advanced from a supply side to a takeup side through a cutting zone in which a workpiece is positioned and to which a cutting liquid, typically distilled water or a liquid medium of dielectric and/or electrolytic nature, is supplied. The wire electrode is continuously advanced conveniently between a pair of guide members to define a linear or straight and continuously traveling wire stretch therebetween for positioning it precisely in a predetermined machining relationship with the workpiece traversed axially thereby. An electrical machining current, advantageously in the form of a succession of time-spaced and precisely adjusted electrical pulses, is applied across a machining gap formed between the workpiece and the traveling-wire stretch to effect electrical discharges and/or electrolytic action and electroerosively remove material from the workpiece. As material removal proceeds, the workpiece is displaced relative to the linear traveling-wire stretch transversely to the axis thereof, typically under numerical control, along a predetermined path to generate a desired pattern of cut or contour in the workpiece. The continuous advance or travel of the wire electrode is effected typically by traction drive rollers disposed at a location between the guide member on the downstream side and the wire takeup means. A desired tension is established in the traveling wire bridged between the guide members typically by providing brake means at a location between the guide member on the upstream side and the wire supply means.
The path along which the workpiece is displaced relative to the linear traveling-wire stretch transversely to the axis thereof is defined empirically so that the resulting machined contour may precisely coincide both dimensionally and in shape with the desired pattern of cut. The tolerance or difference in size between the machined contour and the envelope line of the passage of the machining wire electrode (adjacent to the machined contour) is commonly called "overcut" and corresponds to the size of the machining gap formed between the wire electrode and the workpiece. There is therefore a deviation of size between the path of the axis of the machining wire electrode and the resulting or desired contour in the workpiece which is equal to the radius of the wire electrode plus the machining gap or "overcut".
An attempt in the art to yield a machined contour with high precision, surface finish and efficiency with respect to a desired contour makes common use of a double or multiple step wire-cutting process. In this process, the first step is used to rough-cut a workpiece by displacing the wire electrode relative to the workpiece along a path corresponding to the desired contour but sized to yield a rough-cut contour. The second step is used to remove the difference between the rough-cut contour and the desired contour by displacing the wire electrode relative to the workpiece along a path shifted in the plane of displacement from the first-step path by a finish-cut distance. In the prior art, this latter distance together with the rough size path in the first step has been determined solely empirically. It has been the common practice in the art to choose the finish-step cut size roughly equal to the overcut in the first, rough-cutting step.
I have now found that in such a two- or multi-step wire-cutting operation a certain problem arises due to peculiar dynamic characteristics of the wire electrode in electroerosively cutting along the previously rough-cut contour in the finish-cutting step. In the ordinary electroerosive operation or rough-cutting step in which the wire electrode is advanced transversely in the workpiece, the machining action takes place preferentially along the semi-circular surface of the wire electrode located in the direction of advance. The machining action is accompanied by machining pressure, e.g. discharge pressure and/or expansion pressure of gases produced, and accordingly tends to force the advancing wire electrode back or in the direction opposite the direction of advance in the ordinary or rough-cutting operation. In the finish-cutting step, the wire electrode must be advancing semi-tangentially along the rough-cut contour and there develops an imbalance in the machining pressure on one and the other semi-circular surface sides on the wire electrode with respect to the direction of advance, which produces additionally a wire deflection force which tends to force the wire electrode in a direction away from the workpiece contour being finish-cut and may give rise to finishing inaccuracy. It has been found that this additional wire deflection force develops in spite of a considerable external tension applied to the linear wire stretch bridged and traveling across the cutting zone and increases with the increase in the thickness of the workpiece. Thus, when the trajectory of displacement of the wire electrode in the finishing step is a path simply shifted from that in the roughing step by a distance defined roughly on the conventional empirical basis or without regard to the workpiece thickness, the desired finished precision could not be obtained. The position of the wire electrode may even be shifted far away from the given wire trajectory in the finishing step so that the machined contour in the workpiece deviates largely from the desired contour.